Non-covalent precedes covalent interaction between drug and protein
Small molecules like drugs bind to proteins, e.g. human serum albumin (HSA). The attachment of a drug to a protein occurs first via non-covalent bonds (table 1 and figure 1) (14-16). These are the dominant type of intermolecular forces in supra-molecular chemistry and rely on van der Waals forces, electrostatic interactions, ion pairs, and hydrogen pairs. Even though they are weak individually, their cumulative energies of molecular interactions can be significant. The bindings are formed very rapidly, are reversible and the molar concentrations of the protein [P], ligand [L], and complex [LP] respectively are determined by the dissociation constant: Kd= [L][P]/ [LP]. Thus, the affinity of interaction and the concentrations of drug and protein determine the number of complexes formed.
After an initial non-covalent binding the drug may bind by a covalent bond to a certain amino acid within the protein (figure 1). This feature depends on the chemical property of the drug. Other drugs cannot bind directly, but gain this property by metabolism (13).
A drug or drug metabolite able to bind by covalent bonds to a carrier molecule/protein is called a hapten. Covalent bonds involve the sharing of electron pairs between atoms. The formation of such bonds may take some time as studies with penicillin revealed (17-19). Under optimized in-vitro conditions (pH 10.2), the first bond of a beta-lactam like penicillin G (as penicilloyl or penicillenic acid) to lysine 199 of human serum albumin (HSA) can be observed at 20 min. At physiological pH 7.4 bonds are seen after 60 min or later and the process continues in the following hours. It is not readily reversible. The resulting hapten-modified protein (“adduct”) represents a new antigen, to which an immune response can be developed.
Of note, the drug-protein complexes or adducts based on non-covalent or covalent bindings have a very similar “appearance”, as the location of binding and the orientation of the drug versus protein may be the same in non-covalent and covalent bindings (14). Consequently, an antibody initiated by the covalent hapten-protein adduct may recognize both, the complex formed by covalent bonds and the complex formed by non-covalent bindings (figure 1).